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Introduction to Acids and Bases (Worksheet)

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Name: ______________________________

Section: _____________________________

Student ID#:__________________________

Work in groups on these problems. You should try to answer the questions without referring to your textbook. If you get stuck, try asking another group for help.

“For every complex question, there is a simple answer—and it’s wrong.” H. L. Mencken

Introduction

Originally the terms acid and base referred to taste. The practice of classifying substances according to their acidic (sour) or basic (alkaline or bitter) properties dates back to ancient times. An acid was something with a sour taste, such as lemon juice, and a base was something with a bitter taste, such as tonic water. Today there are three additional categories of taste: sweet , salty and umami . The newest, umami, is specific to mono sodium glutamate (MSG). It is no coincidence that the acid–base properties of compounds are related to taste. Human taste receptors coupled with smell receptors have evolved to interpret certain molecular features as different tastes. Compounds formed from combinations of acids and bases taste salty and are referred to in chemistry as salts. Sweet compounds have characteristics of both acids and bases in the same molecule. We will explore the relationship between molecular structure and acids–bases, and consider water solutions of acids and bases. In water or a water solution, the solution is acidic if the hydrogen ion concentration is greater than the hydroxide (OH - ) ion concentration, the solution is basic if the hydroxide ion concentration is greater than the hydrogen (H + ) ion concentration, and the solution is neutral when the concentrations are equal. Thus the properties of an acid solution are due to the relatively high concentration of hydrogen ion, and the properties of basic solutions are due to the high concentration of hydroxide ions. As do many of the fundamental ideas in chemistry, the acid–base concept dates back to ancient times and derives from everyday observations about substances people encountered. It was centuries later, however, before molecular interpretations were given to these real-life observations. The acid–base concept is a system of classifying chemical substances which permits both the organization as well as the prediction of a vast number of chemical reactions. A substance may be assigned to one our four conceivable categories. It may be an acid or a base, but in addition, it may be both an acid and a base or it may be neither an acid nor a base. Early chemists realized that even among acids and bases, some acids were stronger (more sour) or more basic (more bitter) than others. Thus acids may be further classified as strong acids and weak acids, and bases as strong bases and weak bases.

Three Definitions of Acids and Bases

The Arrhenius definition of acids and bases is the oldest (1884) of the three with which you should be familiar. It derives from Arrhenius’ theories concerning the formation of ions in aqueous solution. It is interesting that he first proposed this idea as a student but his professors considered the idea to be nonsense. As a result he nearly failed to earn the doctorate. He stuck to his convictions, however, and earned a Nobel Prize nineteen years later for this very same insight. An Arrhenius acid is a hydrogen-containing substance which yields hydrogen ions in aqueous solution. An Arrhenius base is a hydroxyl-containing substance which yields hydroxide ions in aqueous solution. It is obvious from the definitions that for a substance to be an acid in the Arrhenius sense, it must have at least one hydrogen atom, and to be a base, it must have at least one hydroxyl group. Moreover, the Arrhenius definitions only apply to the behavior of substances in water.

Brønsted–Lowry

The Brønsted–Lowry definition of acids and bases liberates the acid–base concept from its limitation to aqueous solutions, as well as the requirement that bases contain the hydroxyl group. A Brønsted–Lowry acid is a hydrogen-containing species which is capable of acting as a proton (hydrogen ion) donor. A Brønsted–Lowry base is a species which is capable of acting as a proton acceptor. We see that the Brønsted–Lowry acids and bases include those substances that are classified as Arrhenius acids and bases. A substance which yields hydrogen ions is a proton donor, and a substance containing a hydroxyl group which is capable of yielding hydroxide ions in aqueous solution would also be a proton acceptor. Brønsted–Lowry bases, however, are neither required to contain hydroxyl groups nor to form ions in solution.

The Lewis definition is the most general of the three. It liberates the acid–base concept from its reliance on the presence of any particular element. It focuses on the behavior of the electrons during an acid–base reaction. The importance of the Lewis definition is that it gets at the basis of acid–base behavior and catalogues the largest number of molecules and reactions. A Lewis acid is an electron pair acceptor; a Lewis base is an electron pair donor.

Conjugate Acids and Bases

The concept of conjugate acids and bases is best understood by considering what happens when a substance behaves as a Brønsted–Lowry acid or a Brønsted–Lowry base. By reversing the reaction in which a substance acts as a proton donor, we see that the product is itself a proton acceptor. It is thus a base, or more specifically, it is the conjugate base of the original acid. Similarly, when a substance behaves as a Brønsted–Lowry base, the product is a proton donor. It is thus the conjugate acid of the original base. In summary, a conjugate base is the species that remains after a Brønsted–Lowry acid donates a proton, and a conjugate acid is the species that forms when a Brønsted–Lowry base accepts a proton.

A brief glossary of terms relating the properties of atoms /molecules to acids and bases: Atomic charge is the charge (usually a fraction) that an atom or group of atoms carries when it is in a molecule. All of the atomic charges in a species must add up to the charge on that species. Atomic charges are important because the charge on an atom in a molecule is its optimal charge. In a reaction if this charge is to be increased or decreased, energy must be supplied. Generally, chemical species that are positively charged are acidic; negatively charged are basic. Electronegativity is the ability of an atom to attract electrons to itself in a bond. Thus atoms of high electronegativity will acquire excess negative charge (electrons) and will thus have negative atomic charges. They will tend to be bases. On the other hand, atoms of lower electronegativity will then have lost negative charge—electrons—and will have positive atomic charges. They will tend to be acids. A lone pair or free pair of electrons is a pair of electrons which are in the same orbital. They do not participate in bonding in the molecule. However, they may form a bond with another chemical species that is positively charged, and in so doing act as a base. A functional group is an atom or group of atoms that has a specific arrangement in an organic molecule. The type of functional group relates to certain chemical properties of that molecule. For example, the –COOH group is referred to as a carboxylic acid. This functional group is the source of acidity in organic acids. Another functional group, the amine, has a nitrogen with a free pair of electrons and behaves as a base. pH is a logarithmic unit of concentration. Specifically, pH = –log [H+] ; pH = –log [H 3 O + ]. It is a scale from 0 to 14. Values <7 are acidic; >7 are basic, and a value =7 is neutral. The pH scale was developed by a Belgian brewmaster to control the quality of his product.

The Water Equilibrium (A critical concept in understanding acid-base behavior)

Pure water is mostly made up of water molecules, but it also consists of very tiny quantities of hydrogen and hydroxide ions in equal amounts. These result from the spontaneous, natural autoionization of water:

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We can write an equilibrium constant expression for this reaction in the usual manner. It is assigned the symbol Kw to indicate that it is the equilibrium constant for the autoionization of water:

K w = [H + ] [OH – ] or K w = [H 3 O + ][OH – ] (1.2)

The value of Kw depends on temperature, as does any equilibrium constant. At 25°C, K w = 1.0 x 10 –14 . Since the dissociation of one water molecule yields one hydrogen ion and one hydroxide ion, their concentrations must be equal in pure water. This allows the calculation of each:

[H 3 O + ] = [H+] = [OH–] = = 1.0 x 10 –7 M (1.3)

Water solutions, composed of solutes dissolved in water, do not necessarily have equal hydrogen ion and hydroxide ion concentrations. Such solutions will still follow Equation 18.2, however, and the product of the concentrations of the hydrogen and hydroxide ions will equal 1.0 x 10 –14 at 25°C.

The pH Scale

Working with the tiny numbers associated with hydrogen ion and hydroxide ion concentrations in solutions can be awkward. For convenience, chemists often work with base-10 logarithms, which are referred to as “p” numbers. This allows concentrations to be expressed as numbers that generally range between 1 and 14. If Z is value, then, by definition pZ = –log Z (1.4) When applied to hydrogen ion and hydroxide ion concentration, we have pH = –log [H+] = –log [H 3 O + ] and pOH = –log [OH–] (1.5) Working in the other direction, if pH or pOH is known and hydrogen ion or hydroxide concentration is wanted, [H 3 O + ] = [H+] = 10 –pH and [OH–] = 10 –pOH (1.6)

Weak Acid Equilibria

A weak acid is a hydrogen-bearing molecular compound that ionizes only slightly in water solution. Using HX(aq) as the formula of a weak acid dissolved in water, the ionization equilibrium is HX (aq) H + (aq) + X–(aq) (1.7) H 2 O (l) + HX (aq) H 3 O + (aq) + X – (aq) and the associated equilibrium expression is K a = (1.8) K a is the equilibrium constant for the acid. The concentration of the undissociated acid, HX, is much greater than the concentrations of hydrogen ion and the conjugate base of the weak acid, X–. Therefore, weak acid equilibrium constants have values less than one. The weaker the acid, the smaller the value of the equilibrium constant. Ka values for many common weak acids are available in reference books and general chemistry textbooks. Another important concept arises from the fact that weak acids dissociate only slightly: The amount of weak acid that ionizes is usually negligible when compare with its initial concentration. To illustrate, consider a 0.10 M solution of a weak acid. If this acid is 2.0% dissociated at equilibrium, 0.10 x 0.020 = 0.0020 moles of acid per liter of solution are ionized. Applying the rules of significant figures, 0.10 M – 0.0020 M = 0.10 M of acid remains undissocated, which is the initial concentration. Thus, you can see that the amount that ionizes is not significant, and the initial acid concentration essentially is unchanged by the dissociation. The concentrations of hydrogen ion and conjugate base of the weak acid cannot be neglected in weak acid equilibria. In fact, determining these concentrations is frequently the goal of theoretical calculations and experimental investigations of weak acid solutions. If the weak acid solution is not combined with any other compound, [H 3 O + ] = [H + ] = [X – ] because each comes from the same source, the dissociation of HX (aq).

• Acids and Bases: Worksheet #1
• Acids and Bases: Worksheet #2
• Acids and Bases: Worksheet #3

Outside Links

• http://www.zoology.ubc.ca/~auld/bio3...ory_taste.html
• Tutorial: http://www.science.ubc.ca/~chem/tutorials/pH/

Contributors

• Ron Rusay, Department of Chemistry

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Acids and Bases

Acids and Bases introduces students to pH levels and how to tell the difference between an acid and a base. Students will learn to define and identify both types of substances and explain how they differ. They will also discover why this type of information is important to know.

In the “Options for Lesson” section of the classroom procedure page, you will find several suggestions to add to your lesson delivery. One option is to use litmus paper to test the pH level of a number of substances, including students’ favorite drinks that they bring to class.

Description

Additional information, what our acids and bases lesson plan includes.

Lesson Objectives and Overview: Acids and Bases explores acidic and basic substances and how to measure their pH levels. Students will learn the traits of both an acid and a base. They will be able to explain how the two substances differ. They will also learn what pH means and how it helps scientists measure the hydrogen levels of many substances. This lesson is for students in 5th grade and 6th grade.

Classroom Procedure

Every lesson plan provides you with a classroom procedure page that outlines a step-by-step guide to follow. You do not have to follow the guide exactly. The guide helps you organize the lesson and details when to hand out worksheets. It also lists information in the yellow box that you might find useful. You will find the lesson objectives, state standards, and number of class sessions the lesson should take to complete in this area. In addition, it describes the supplies you will need as well as what and how you need to prepare beforehand. You will need a number of supplies for this lesson—white construction paper, scissors, glue, and colored pencils. If you can, try to obtain litmus paper and a variety of liquids to test their pH values.

Options for Lesson

The “Options for Lesson” section on the classroom procedure page provides a number of extra suggestions for additional activities or ways to alter different parts of the lesson. One suggestion is to get litmus paper and about 10 to 12 substances for students to test and identify as either acids or bases. Another idea is to assign the practice worksheet before the activity rather than after. Students could also bring their favorite drink to class and test whether it is basic or acidic according to its pH level. Another option is to invite a chemist to the class to discuss more about acids and bases with the students.

Teacher Notes

The teacher notes page includes a paragraph that provides a little more information about the lesson or more ideas. It suggests obtaining litmus paper if possible as doing so will greatly enhance the lesson plan and students’ learning about acids and bases. You can use the blank lines on the page to write down any other ideas or thoughts you have prior to the lesson.

ACIDS AND BASES LESSON PLAN CONTENT PAGES

The Acids and Bases lesson plan contains three pages of content. The first page explains why all drinks and some foods have a specific taste. Some are more sour while others are on the bitter side. Examples of sour foods or liquids include buttermilk, lemon juice, and some candies. Other things taste bitter. The foods and drinks that have a sour taste are acidic, and the ones with a bitter taste are more basic.

The lesson provides lists containing facts about acids and bases along with examples of each. Students will learn that the word acid comes from a Latin word that means sour— acere . Some acids are natural, meaning they exist in nature. People often drink liquids that contain these acids. An acid is a molecule that splits apart in water and releases hydrogen ions.

The stomach contains hydrochloric acid, which it uses to digest food and kill disease-causing germs. Batteries of all kinds also contain a type of acid. That includes the batteries you put in toys or remote controls as well as the ones that go inside a real car. Many drinkable liquids also contain acids, like lemon juice, orange juice, and tomato juice.

Bases, on the other hand, have a bitter taste and a soap-like texture. They just happen to feel soapy when rubbing between the fingers. These substances also occur naturally. A base is a molecule that splits apart in water and releases hydroxide ions, which reduces the number of hydrogen ions.

Examples of bases include soapy water, milk, bleach, and Milk of Magnesia. In the body, the pancreas contains a basic substance that helps with digestion. People use bases in household cleaning products and crop fertilizing.

The Indicators

The first person to define these two types of substances was a chemist named Svante Arrhenius in 1887. It would be impossible to determine whether a substance is acidic or basic by tasting every liquid in nature. It would also be dangerous, after all. Instead, there is a special type of substance scientists use to determine whether a liquid is acidic or basic in nature. This special substance is called an indicator.

There are multiple indicators that help scientists with this task. Indicators change color depending on whether the substance is an acid or a base. Three naturally occurring indicators include litmus, turmeric, and China rose. If someone dips any one of these into a liquid, the liquid will change color, which indicates its base or acid level.

Students will discover that the indicator people most commonly use is litmus. Its natural color is purple. When an acidic solution touches it, it will turn red. On the other hand, it will turn blue if someone dips the litmus into a basic solution. Litmus comes from organisms called lichens, which come from both fungi and algae. In addition, litmus can come in either a paper form or as a solution.

The final page of the lesson discusses pH and how this helps determine the a substance’s acidic or basic level. The lesson shows the pH spectrum that all substances fall on, ranging from 0 to 14. The most acidic substances have a low pH, while the most basic substances have a high pH. This means that while two solutions are acidic, one might be more acidic than the other. For instance, black coffee is acidic, and so is the gastric acid in the digestive system. Black coffee, however, is less acidic than gastric acid and thus has a higher pH level.

A pH scale, then, measures how basic or acidic a solution is. It stands for potential of hydrogen, meaning that it measures how many hydrogen ions are in a specific solution. The more hydrogen ions, the more acidic a solution. The more hydroxide ions, the more basic a solution. Liquids that have a pH between 0 and 7 are acidic, and those with 0 are the strongest acids. Bases have pH scores between 7 and 14, with the score of 14 indicating the strongest base. A score of 7 indicates that a solution is neutral, meaning that there is a balance between both the hydrogen and hydroxide ions. Pure water is a neutral substance with a pH of 7.

Acids with a low pH and bases with a high pH are highly dangerous and very reactive. In fact, they could corrode or burn a person’s skin. But people can use the two types to neutralize each other. When a person’s stomach has excess acid, for example, they could drink milk or use a medicine called Milk of Magnesia to neutralize the acid and make their stomach feel better. Students will be interested to learn that toothpaste actually does something similar! Food particles decay in the mouth and increase the acidity levels, leading to tooth decay. Toothpaste helps neutralize the acid and thus prevent tooth decay.

ACIDS AND BASES LESSON PLAN WORKSHEETS

The Acids and Bases lesson plan includes three worksheets: an activity worksheet, a practice worksheet, and a homework assignment. Each worksheet will help reinforce students’ comprehension of the concepts and material they learned throughout the lesson. You can refer to the guide on the classroom procedure page which outlines when to hand out the worksheets.

ACIDIC TO BASIC ACTIVITY WORKSHEET

For the activity, students will cut out 12 pictures of different substances. Using what they learned during the lesson, they will have to order them from most acidic to most basic. They will glue the images onto a piece of construction paper that shows the pH scale.

ACIDS AND BASES PRACTICE WORKSHEET

There are two parts of the practice worksheet. The first part requires students to match statements to the correct term. There are 15 statements and terms to match up. The second section requires them to compare two liquids to each other. There are 10 pairs of liquids to compare in this section.

WHAT DID YOU LEARN HOMEWORK ASSIGNMENT

Students must answer 20 questions that relate to the information they learned throughout the lesson. If you want to allow students to use the content pages for reference, you may do so. You may, however, wish to test their memory of the lesson material instead.

Worksheet Answer Keys

The final pages of the lesson document are answer keys for the worksheets. The activity answer key displays where each liquid should go and orders them from most acidic to most basic. (In the bases section, you will follow from left to right and top to bottom.) The answers on the practice and homework worksheets are in red. For the most part, students’ responses should match exactly. However, a few questions on the homework assignment may include some variation due to the nature of the question. If you choose to administer the lesson pages to your students via PDF, you will need to save a new file that omits these pages. Otherwise, you can simply print out the applicable pages and keep these as reference for yourself when grading assignments.

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